Yep, the 4.6 mustang motor will fit in a Classic. Ian has already done one but he did have to trim the tops of the foot boxes to squeeze it in since it's a wide motor.

Note the original Robnel chassis was made to take a 429 big block but when they went for the 4.6 motor had to change it and widen the bar work in the engine bay to fit the tubby little 4.6 in. I think they also set the 4.6 motor a little bit further forward as well.

The guys in the US are getting good numbers out of these Quad Cam motors with a bit of forced induction.

Eagle have got a special on at the moment for new 4.6 crate motors with a new tremec 5 speed behind it. $AU9,900 from memory. It's not the supercharged version but I reckon that's a pretty good deal for a brand new motor and box.

Find yourself an engineer for the project before you invest in a motor. The Vic laws may be changing to still allow the 4.6 for a while longer but your engineer will know for sure.